Wind turbine generators (hereinafter ‘wind turbines’) of commercial mega-watt scale comprise a rotor which is mounted for rotation atop a tall tower on a main shaft held on a nacelle. The nacelle also houses the main operative components including a gearbox, if provided, and generator, converter etc. In the erection of wind turbines it is necessary to lift the various components of the turbine, including tower sections themselves as the tower is erected, and the various operative components. As the size of wind turbines grows ever larger these components become larger and heavier, and more challenging to erect and service.
Commonly, stand-alone service cranes are used for the lifting of major components on turbine erection. These have the disadvantage of needing to be significantly taller then the tower, and require significant time to erect and dismantle, needing a large amount of transportation equipment, and may suffer problems of access in difficult to reach places, with the result that they are an expensive and somewhat inflexible solution.
The turbine typically also has an internal crane within the nacelle which is suitable for the lifting of relatively small components. Such cranes are not however of sufficient capacity, dimension or positioning to be able to handle major components including turbine blades or gearboxes or main bearings.
It has been proposed to provide dedicated cranes which can be hoisted up the tower (or in certain versions can self-climb up) to an appropriate position on the tower for the task in hand, or up to the position of the nacelle, and which is secured to the tower and/or nacelle in a working position where it is able to handle major turbine component installation, replacement and repair.
In Applicant's WO2009/080047 the contents of which are hereby incorporated by reference there is disclosed a dedicated crane having a main body from which extends a work platform, with a pair of gripping arms which serve to clamp tightly around the tower when in the working position. The crane is hoisted through the use of an intermediate pulley assembly, whereby the small capacity internal nacelle crane is used to hoist this intermediate pulley assembly up to the nacelle, and this intermediate pulley assembly then used to hoist the crane body, with hoist motors on the crane, or on the ground or on a transporting truck, providing the drive for the hoisting operation. The crane is carried to the site on a truck and hoisted from the truck, with a guiding cable secured to the crane and being played out from a drum on the truck as the crane is hoisted, serving to guide the crane and prevent undue swinging of the crane which might otherwise cause damage to the crane and tower.
The hoisting of the crane to the nacelle involves particular challenges. Firstly, lifting the crane off the truck or other mode of transport from which it has been transported to the site presents a challenge as, depending on its design, the crane is typically transported in a horizontal orientation and must be raised to its vertical upright orientation before hoisting. This change in orientation requires care as the crane is in general designed for handling in its vertical working orientation. Moreover, hoisting of the crane up the tower involves a large vertical distance of travel (current generation towers for large wind turbine models may be 80 to 120 m in height and are growing as the drive for ever-larger turbines continues). Towers are typically slightly conical showing a significant reduction in diameter up the tower. Finally, as the crane approaches the nacelle the direction from which the tension on the lifting point arises changes quite significantly, with implications for the balance and stability. Controlling the hoisting of the crane is of the upmost importance in order that the crane may be safely raised without damage to crane, tower or risk of danger to personnel, and this may be in weather conditions which are less than ideal The present invention seeks to provide a crane structure which facilitates safe lifting.